The invention relates to a process for the recovery of a gaseous phase from a liquid fluid by at least partial evaporation of the liquid fluid or at least one of the components contained therein or by setting free a component formed by decomposition. The process is implemented in a modular falling-film evaporator comprising stacked vertical or inclined plate-type modules that form gap-type evaporation chambers between the adjacent modules.
The device according to the invention is a reactor that can be operated on an industrial scale, especially on a scale in terms of tonnes.
Falling-film evaporators, which can be designed in the form of vertical tube bundles or vertical plate stacks, have manifold uses in the chemical processing industries. The advantage of falling-film evaporators as compared to conventional forced-circulation or natural-circulation evaporators is the low superheating of the liquid. Whereas in the case of falling-film evaporators the liquid film flows over the surface exclusively by gravity—reference is made to a typical layout in U.S. Pat. No. 5,203,406—thin-film evaporators are used for especially thermolabile substances, mechanical means being employed to create a thin film on the evaporator surface, the mass and heat transfer being improved by turbulence in the film—U.S. Pat. No. 6,173,762 being referred to as an example. The following examples for the application of falling-film evaporators can be cited: Removal of organic solvents from solutions in order to enrich the thermolabile products dissolved therein and to recover the solvent; concentration of fruit juices (food industry); winning fresh water from salt water or brackish water.
The falling-film evaporator according to U.S. Pat. No. 5,203,406 comprises a package of essentially vertically arranged heat exchanger plates spaced apart by a definite distance, every second space between the said plates forming an evaporation chamber. The structuring of the evaporator plates cannot be deduced from this document.
It is known from U.S. Pat. No. 6,173,762 B1 to structure the film-wetted surface of a heat exchanger tube with fins and grooves for the purpose of improving the heat and mass transfer. The height of the fins is specified as 0.2 mm to 0.8 mm, and the number of fins is specified as 900 to 1100 per meter.
In conventional falling-film evaporators, the spraying density and film thickness may not fall below certain minima, namely around 1 mm in the case of falling-film evaporators and around 0.5 mm in the case of thin-film evaporators, because otherwise film interruptions, dry running and undesirable hot spots may occur, possibly resulting in undesirable decomposition phenomena when concentrating a liquid fluid that contains a thermolabile component.
DE 100 36 602 A1 teaches a modular micro-reactor for the performance of reactions between a liquid and a gaseous reactant. This micro-reactor comprises a stack of any number of vertical or slanted fluid guide plates, the same number of intermediate plates and gap-shaped reaction chambers formed between two adjacent plates. The fluid guide plates feature parallel micro-channels in which a liquid can flow by gravity and/or capillary forces in continuous capillary filaments and come in contact and react with a gas. The fluid guide plate can be coated with a catalyst and can come in contact with a heat transfer fluid on its reverse side. The reaction mixture that forms in the process is withdrawn at the lower end of the fluid guide plates via collecting channels. The disadvantages of the micro-reactor according to DE 100 36 602 A1 are the many feed and discharge lines, collecting and distribution channels, pressure loss barriers and special slopes and the resulting highly complicated technical construction. Although the modular construction can improve the throughput to a certain extent, no proposals are made in the document as to how the miniaturised reaction system has to be modified in order to make it economically suitable for commercial-scale applications. Proposals for using the micro-reactor as an evaporator cannot be deduced from the document.
WO 02/18042 A1 describes a further commercial-scale reactor for performing reactions between at least two fluid reactants. The reactor comprises a stack of essentially rectangular vertical wall elements with an integrated cooling device and gap-type reaction chambers between the adjacent wall elements. Although the wall elements can be structured and coated with a catalyst, nothing can be found in the document neither with regard to the structure consisting of a set of parallel micro-channels nor with regard to the reactor being usable as an evaporator.
DD patent 246 257 teaches a process micro-equipment consisting of a stack of individual small substrate plates with worked-in hollows, the small plates being covered to form closed channel-shaped cavities. This micro-reactor contains channel-type reaction chambers, in which not only chemical reactions but also evaporation processes can take place. Such a micro-reactor is suitable for use as a reaction vessel in chemical micro-analysis but not for use as an evaporator for a commercial-scale process (in terms of tonnes).
DE published document 1 667 241 describes a reactor for chemical reactions comprising a stack of plate-type modules which may, if required, be coated with a catalyst and further comprising gap-type reaction chambers formed by the modules including spacers for delimiting the reaction chamber sizes and inlet and outlet devices. The spacing between the plates is in the order of 0.001 and 6.3 mm, so that the effective reaction chamber can be enlarged as compared with micro-reactors. The surface of the plates on which the liquid film flows by centrifugal force may be etched, but nothing can be found in the document with regard to the presence of parallel micro-channels. Although the plate stack can be temperature-controlled as a whole, nothing can be found in the document with regard to controlling the temperature of individual plates, so that this device is not directly suitable for commercial-scale use.
The reactor according to U.S. Pat. No. 5,811,062 comprises several laminates arranged above each other with microstructured elements therein, such as micro-channels in particular. By interconnecting a plurality of such micro-reactors, reactions can be performed on a macro scale. The dimensions of the micro-components are in the order of 1 μm to 1 cm. The width of the grooves in the laminates forming the channel covers is cited as being 1 μm to 1 mm; insofar as a gap is present above the grooves, the gap width is said to be less than 100 μm and especially less than 10 μm. Although the reactor can be used as an evaporator, nothing is contained in the document in the way of a concept to arrange the laminates with the micro-structures, i.e. the reaction channels, in any other way than vertically. The reactor is not used as a falling-film evaporator. Moreover, it is a disadvantage of this reactor that the size of the grooves is such that, even if there is a small space above the grooves, evaporation of a liquid flowing through the grooves is only possible within extremely narrow limits.
EP patent 0 688 242 teaches an integral structure for chemical processing and production comprising a plurality of interconnected small plates forming at least one three-dimensional continuous loop-type channel between adjacent plates and at least one inlet and outlet port for the substances to be circulated. This modular micro-reactor may also feature channels between adjacent plates for a heat-transfer fluid. This micro-reactor, too, is unsuitable to be enlarged indefinitely by connecting many reactors in parallel and/or by increasing the number of plates within a stack, in order to permit a process to take place on a commercial scale. No concepts can be derived from this document with regard to modifying the micro-reactor in such a way that it becomes suitable for the use as a falling-film evaporator.